Ceramic reinforcement in metal matrices improves the properties or functional characteristics of various metals and alloys. Chopped or continuous fibers, whiskers, or particulates can be used as reinforcement matrix metals to enhance the specific strength (i.e. strength/density), specific modulus (i.e. modulus/density), and the temperature service capabilities of the composites. Improvement in the specific strength is achievable both by reducing the density and by increasing the absolute strength and modulus through the introduction of the ceramic reinforcement.
The result is typically a composite providing a significant weight reduction for components having critical strength or stiffness requirements. For example, a metal matrix composite containing 80 volume % aluminum and 20 volume % silicon carbide has a stiffness comparable to steel, but is considerably lighter. Furthermore, the composite has improved corrosion resistance over steel.
Metal matrix composite research has focused on the development of aluminum based composites using boron, borsic, graphite, or silicon carbide reinforcement in particulate, continuous fiber, or discrete fiber forms. Continuous fibers offer the potential of highly anisotropic properties in the composite by aligning the fibers in primarily one direction. Unfortunately, the off-axis properties of these composites have proven to be quite low. Discontinuous or discrete fibers, however, offer greater potential for tailoring the properties of the composite. For example, by cross-rolling a SiC-Al composite, the composite can possess nearly isotropic properties, while the same composite may be highly anisotropic if prepared by a multiple extrusion process or if worked with only unidirectional rolling. The degree of stiffness anisotropy can be controlled over a wide range.
Forming composites with continuous or very long fibers often requires highly specialized fabrication techniques to avoid (1), fiber breakage, (2) fiber bunching, (3) nonuniform fiber/matrix interfacial bonding, or (4) void concentrations. Whiskers or particulates are more readily used, particularly in powder metallurgy, casting, hot extrusion, rolling, and forging. Machining, drilling, grinding, joining, and other operations are also more readily accomplished with composites having discrete or discontinuous fibers, since the properties oftthe composite are not as severely linked to the continuity of the fiber.
When using powder metallurgy to fabricate composites, the metal matrix powder is blended with the fiber and is cold pressed to form a green compact structure. The green structure is then vacuum compacted or isotactically pressed at elevated temperatures and pressures to cure the green structure and to achieve full density in the composite. Full density is necessary to ensure the integrity of the article and to attain the necessary mechanical properties. Unfortunately, the high temperatures required for vacuum compaction to full density can lead to adverse reaction between the fibers and matrix metal, especially for SiC fibers in reactive metals like aluminum and titanium. Such reaction affects the integrity of the composites and their mechanical properties. Secondary phases, such as carbides, borides, silicides, or nitrides, can be formed in these reactive composites, and are predictable based upon thermodynamic considerations. Reducing the deleterious reaction between the fibers and matrix is a necessary improvement to metal matrix composite technology.
U.S. Pat. Nos. 4,073,648 (Volin et al. ) and 3,976,482 (Larson) disclose inducing strain energy in prealloyed metal powder to improve thermoplasticity of the powder used in specialty superalloys, particularly in powder metallurgy (P/M).
Methods for forming metal matrix composites are illustrated in U.S. Pat. Nos. 3,546,769; 4,060,412: and 4,259,112.